There almost certainly is life elsewhere in the universe. There is no reason to think that conditions and events that led to life on earth are unique, and the universe is a ridiculously vast place. So the odds strongly favor that there must be many occurrences of life out there. There are interesting sub-questions, however – how common is life, how common is complex life, and how common are technological civilizations? Is the universe teaming with bacteria and fungus and little else, or are the stars buzzing with spacefaring races of every description?

The problem with trying to answer this question is that we have an N of 1 – Earth is the only example of life in the universe that we have. We may find examples of life elsewhere in our own solar system (Mars, Europa, Titan, and Enceladus are the current prime candidates), but that will only give us a tiny bit more data. Life elsewhere in our own solar system (especially Mars) may have been seeded from Earth or vice versa, and so we may find life on Mars but still only have evidence for a single life origin in our solar system.

We may also find multiple independent life origins in our solar system, and that would be extremely cool, but would still only answer one of the three questions above. That would tell us that the origin of some kind of life is likely common, and can occur under a variety of conditions, but would not tell us how common complex life or civilizations are.

We are still left with the problem that we only have one example of a solar system (our own) with life, and we do not yet have enough data to tell us how typical our solar system is (and also how typical the Earth as a planet is). In order to evolve complex life which then has enough time to develop into a technological civilization, stability over a long period of time is necessary. On earth it took about 4 billion years. Even if that is slower than average, it seems very likely that billions of years of stability are necessary.

The question then becomes – what are all the factors that led to sufficient conditions and stability on Earth that allowed for the evolution of complex life and us? Astronomers have identified many such factors, and we are starting to gather information about them. One such factor is the arrangement of planets in our solar system. Small rocky worlds are relatively close to the sun, around the Goldilocks zone where liquid water on the surface is possible. Three planets are potentially in that zone, although Mars was too small to hold onto its atmosphere long enough, and Venus suffered from a runaway greenhouse effect making it too hot.

Large gas giants (Jovian planets) are farther out. These large planets (especially Jupiter) act like sentinels, shielding the inner solar system from comets and other debris that could potentially impact those worlds and cause mass extinctions. Life can obviously tolerate some impacts, but too many would likely hamper life’s evolution.

NASA scientists have recently identified another feature of our solar system that may contribute to this stability – the asteroid belt. They write:

“To have such ideal conditions you need a giant planet like Jupiter that is just outside the asteroid belt [and] that migrated a little bit, but not through the belt,” Livio explained. “If a large planet like Jupiter migrates through the belt, it would scatter the material. If, on the other hand, a large planet did not migrate at all, that, too, is not good because the asteroid belt would be too massive. There would be so much bombardment from asteroids that life may never evolve.”

The location of the gas giants seems to be important. In some systems they migrate very close to their star – so-called “hot Jupiters.” In such systems inner rocky Goldilocks zone planets could not survive with a stable orbit. Martin and Livio are now saying that, not only do we need a system without a hot Jupiter, complex life requires a Jovian planet just outside the snow line that has migrate a little, but not too much. The snow line is the distance from a star at which volatile substances can exist without evaporating, so you can have icy worlds with frozen water and other compounds. Jupiter is right outside our snow line, and the asteroid belt is just inside. Jupiter is responsible for the asteroid belt forming as it did, and not forming into another planet.

The weakest link in their argument is that if a system has no asteroid belt (because it has no Jovian world just outside the snow line) then the lack of such a belt would reduce asteroid impacts on a potential life-bearing world. Such impacts deliver water and organic materials to the surface of the planet, and also a steady stream of impacts (but not too many) provides an occasional kick in the pants to ecosystems, furthering their evolution. These are not unreasonable assumptions, but they are just assumptions and are not empirically verified to the point that we can use them as solid premises.

Other systems may have similar rates of asteroid or cometary impacts without such an asteroid belt. It’s also possible that the range of such impacts compatible with life is large. Another factor is the Moon. The Moon is another factor that may have contributed to the Earth’s stability, and one role that it plays is to further shield the earth from potential impacts.

There are many variables that could affect the long-term conditions on a potentially life-bearing planet. I think it’s too early to say how they will all shake out in a typical system. The ultimate goal is to come up with an estimate of the percentage of systems that could harbor complex life. We are not yet close to this. We may also need to expand our concept of potentially life-bearing worlds. What about moons of Jovian planets, for example?

While we are gathering more and more information about the composition of other solar systems, our data is still hugely biased by our search methods. Larger planets closer to their stars are easier to find than smaller farther out worlds.

More importantly, we currently have no data on the existence of life of any kind outside our own solar system, so we can only guess at how the variables of solar system composition we are finding affect the probability of complex life forming. We have no prospects of directly finding such life anytime soon. We may see oxygen in the atmosphere of distant worlds, and that might strongly imply the presence of life, but we won’t know for sure. Unless SETI finds and starts downloading the Encyclopedia Galactica we have no hope of surveying our galaxy for life anytime soon with any extrapolation of current technology. A truly game-changing technology (and underlying physics) would need to be discovered.

But I may be wrong. Future scientists often prove to be more clever than futurists imagine.

17 Responses to “How Common Is Life in the Universe”

I’ve heard the Jupiter comet sweeper argument before, but I don’t understand how that could be such a big deal. Considering that the planets are all on one plane, anything that comes at Earth from the “bottom” or the “top” wouldn’t get swept away by Jupiter. Even if you say one-fourth of the year Earth is close enough to Jupiter that it could influence oblique approaches to the Earth, that leaves plenty of time for the Earth to be less protected. Does this objection make sense?

Given what I know, it seems pretty unlikely to me that there are any nearby technological species, and by “nearby,” I think I mean within our galaxy. Most of our planet’s life was spent in single cell form, and the time we’re going to spend as a technological species could likely be very short with all the doomsday weapons we stockpile. Presumably, advanced aliens might face similar crises. Of course, it could very well be that advanced civilizations that last long enough become stable on larger timescales, raising the chances, but I wouldn’t bet on it.

Add on all the hypothetical necessities like a stabilizing moon around the life bearing planet, an outer jovian planet, and so on, and I’m even more skeptical.

Mark – but most objects that would threaten the Earth are also in the plane of the solar system, not coming from above or below. At some point at their journey to the inner solar system they are likely to encounter Jupiter’s gravity and either be swallowed up or shot out of the system.

Thanks, I went to Wikipedia “Comet” to get this: “Most short-period comets (those with orbital periods shorter than 20 years and inclinations of 20–30 degrees or less) are called Jupiter-family comets. Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets. As of 2012, only 64 Halley-type comets have been observed, compared with nearly 450 identified Jupiter-family comets.”

I agree about your weakest link thought. It strikes me that if the hydrogen and o2 exist on the planet, then it just needs conditions, not asteroids to get water.

In fact all of the theory rides the assumption, as you noted, that the only way to evolve life is the way earth did.

Considering chaos, it just feels to me, very unscientifically, that there would be more than one way to crack this egg. I mean just because we are the delicate creatures that need a soft and warm world, doesn’t mean that life couldn’t develop in a world that we could not survive in.

I’m certainly open to weirdness like silicon-based life and such, which would have different needs (Though regular meteor/comet bombardment would probably still be bad for them). I don’t remember where, but I vaguely remember some weird concept of life that would form around neutron stars, powered by the electromagnetic forces. Can’t say if the show was serious or if they were just trying to woo the audience.

Given what I’ve heard of abiogenesis experiments succeeding at producing complex organic molecules under many conditions and one clip of Tyson talking about how the elements in our body are pretty close to the proportions they make up in the universe as a whole, I suspect simple organic life is probably pretty abundant in the universe.

How common is life? I suspect that it’s probably extraordinarily rare (that is, you’d have to look at an exceptionally large number of star systems and planets before you found another one with life*), but in a universe with on the order of 10,000 billion billion stars, the number of planets in the universe with life on them is probably quite large.

Consider that if there is only one planet with life for every one hundred billion stars (which would make life extremely rare), that would still mean there would be on the order of 100 billion planets with life in the universe, though probably only around one per galaxy.

Life is likely both abundant and rare at the same time, depending on how you look at it.

* At least life that does not have a common origin with life on Earth. Discovering life on Mars that shared a common origin with Earth’s organisms wouldn’t change my opinion on this matter at all. Finding life on Mars with an origin distinct from that of earth’s life would cause me to reverse my opinion.

If I was to bet money on it In would say there have to be many civilisations more advanced technologically than us. Why? Just sheer enormously, stupendous numbers and for me we seem to have an over inflated opinion about how well we have done with what we have. We could have done better and probably should have and with better conditions it is likely we would have too.

Our relatively large satellite does more than sweep life killing asteroids from the sky. It stabilizes the earths axis which in turn helps keep climate stable. There is also the relative tidal effect due to its large size. This probably contributed to the evolution of life outside of the sea. That is just one small part of the complexities of finding a similar world as a livable planet.
Life and evolution of “advanced life on earth” is also subject to time. Given our N-1 example, we are just a “Planck Length” second of the history of the universe. Given a finite life to our star, and current lack of faster than light travel we”humans” in some form will have a short finite existence on a galactic scale. It took us one third of our stars life to appear and if we don’t figure out how to leave or prolong the life of our solar system we will only be around a tiny fraction of the remaining life of the universe. That is also assuming we don’t kill ourselves some other way. Even if we make maximum solar life of our system it will be unlivable well before the collapse of our star into a dwarf star. My point is that I am sure that there is life somewhere in the universe but finding even simple life is a lottery ticket win within our short lives. Little chance we will ever see evidence of “Advanced” life resembling ours. If we say there are 1.6 trillion Goldilocks’s earths in our “local” part of the universe, it is entirely possible that 2/3 are either in early formation or late life resulting in unlivable conditions. Given factors of time, distance and our short lifespan odds weigh heavily against us. For all we know we are the fruit flies of the universe with ridiculously short lifespans.

The Drake Equation is an estimate of the probability of there being a technological civilisation that releases detectable signals into space.

The Drake equation states that:

where:
N = the number of civilizations in our galaxy with which communication might be possible;
and
R* = the average rate of star formation per year in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
fℓ = the fraction of the above that actually go on to develop life at some point
fi = the fraction of the above that actually go on to develop intelligent life
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space

This is all very well, but the thing is we have no real idea of what any of the actual numbers pertaining to the above equation may be.

As Steven has said, we have an N of 1. While this may change tomorrow, it may also never ever change. That would be quite sad.

Considerable disagreement on the values of most of these parameters exists, but the values used by Drake and his colleagues in 1961 were:[8][9]
R* = 1/year (1 stars formed per year, on the average over the life of the galaxy; this was regarded as conservative)
fp = 0.2-0.5 (one fifth to one half of all stars formed will have planets)
ne = 1-5 (stars with planets will have between 1 and 5 planets capable of developing life)
fl = 1 (100% of these planets will develop life)
fi = 1 (100% of which will develop intelligent life)
fc = 0.1-0.2 (10-20% of which will be able to communicate)
L = 1000-100,000,000 years (which will last somewhere between 1000 and 100,000,000 years)
Drake states that given the uncertainties, the original meeting concluded that N ≈ L, and there were probably between 1000 and 100,000,000 civilizations in the galaxy.

The above info comes from that “wonderful source of scientific information”, Wikipedia.

Given that, as of this morning we still have an N of 1, I still believe the above estimates can be nothing much more than pure speculation.

Even though I SO want to believe there are other civilizations out there, I think it’s unlikely. I’m pretty sure that the universe is teeming with micro-life, but intelligent life? not so sure. Intelligence is just one of the myriad ways an organism evolves to deal with it’s environment, not the teleological end goal that it’s sometimes made out to be. Amongst our 50 million or so organisms only one (us) developed high end smarts.

“There almost certainly is life elsewhere in the universe”. This is just a guess — we don’t have an N of 1, we have an N of 0. The solar system we know about is not a random one, but one that has life in, so its existence doesn’t tell us anything about the probability of life other than that it is nonzero. If the probability of life is 10^{-100} on a given planet, then there will still be intelligent life on the planet that we live on.

As a result there is no argument that says there is likely to be other life in the universe, since we have absolutely no way of estimating the quantities in the Drake equation, which lends a spurious air of precision to what is basically just an exercise in handwaving.

The most important thing, when life in the Universe is concerned, is not to jump to any conclusions based on fragmentary data.

However, to say that we ‘just don’t know’ would be equally unfair and ultimately untrue. There is a lot of data that could help drive the investigation of various hypotheses. And plenty of room for the unexpected, of course.

The ongoing astrobiological revolution is based on integrating the knowledge gained by various disciplines investigating various fields by various methods and applying some creative (but highly critical) thinking to it all. In this sense, it becomes the most exciting and gratifying scientific endeavour of all times.

But if we are to jump to our feet and shout every time there is a new piece of the puzzle brought to the table (such as ‘methane on Mars’, or ‘no methane on Mars’) and be drowned under a vast mountain of senseless media sensationalism, than crucial points will likely begin to fade from the picture. So will the interest of serious people for the, otherwise fascinating, subject.

Specifically, since this article mainly discusses Rare Earth Hypotheses (REH), which include, but are not limited to ‘Rare Jupiter’, ‘Rare Moon’, and ‘Rare Asteroid Belt’, I would like to bring to the attention of this community a recently published book. It is titled “The Astrobiological Landscape” and one chapter is specifically devoted to discussing REH. It is the most clear, concise, comprehensive and well-written treatment of the subject that I personally have had the chance to encounter so far (although I admit there may be some ‘observation selection effect’ at play).

In the same book you will also find highly enlightened review of other astrobiological ‘big questions’ and the underlying philosophical foundations – in brief, I highly recommend the book. I also apologize to all who are already aware of it, or have read it.

An often over looked factor in discussions of this kind is distribution of life in the universe and how it might be effected by interstellar migration. It could be that even in a universe where life seldom emerges and where that life seldom develops advanced enough technology to travel to nearby stars, that the total population of the universe would be quite high due to this sort of technological panspermia. Once a civilization passes through the bottleneck it would become tenacious enough to persist almost indefinitely due to the low probability of interstellar disasters and by sheer numbers. In a universe of this kind, we might expect to see a kind of large scale clumpiness where whole galaxies or even galactic clusters are populated by life which originated from a single point, while other areas would be devoid of life. There would be the equivalent of urban areas, rural areas and wilderness. Given enough time, this sort of universe might mature into a more homogeneous one, but with most life stemming from a handful of planets. This alludes to the Fermi paradox, which isn’t really a paradox because due to our limited knowledge, we could be right in the middle of a cosmic “downtown” and not even realize it. Alternatively, if we’re in the desert, it might be very difficult for SETI searches to turn up anything, even with improved methods; because of the great distances of these “cities” and even if the populated areas are quite large.

I echo what a few have already said. Based on extremophiles found here on Earth, I think life is probably relatively common in the universe. Perhaps even “complex” (i.e. multicellular) life. What I am far less certain about, is how common intelligent life is. It doesn’t seem to me that intelligence is that huge an evolutionary advantage, especially in terms of its biological cost. A number of different events have brought human to near extinction, and intelligence really didn’t help much (asteroids, ice age, pandemic). At this point in our intelligence development, we like to think our intelligence would help with survival, and that is probably true. However not sure how true that is for most of our history on the planet.